Generally, in the electroacoustic apparatus including the microphone and the speaker, there is a case where howling is caused by an acoustic feedback loop which is formed when a sound output is from speaker returns to the microphone.
A conventional howling suppression device suggested for suppressing howling is a device which suppresses howling by extracting a howling component included in an input signal and subtracting the extracted howling component from the input signal (for example, see Patent Literature 1).
FIG. 12 is a block diagram showing a conventional howling suppression device 1000 described in Patent Literature 1. As shown in FIG. 12, the conventional howling suppression device 1000 includes: an input terminal 1001 into which a signal is input; a delay unit 1002 which delays the input signal; an adaptive filter 1003 which extracts a howling component from the input signal; a subtractor 1004 which subtracts, from the input signal, a signal output from the adaptive filter 1003; and an output terminal 1005 which outputs a signal with which howling is suppressed.
Next, an operation of the conventional howling suppression device 1000 described in Patent Literature 1 will be described.
A signal is input into the input terminal 1001 from the microphone which is not shown in the figure. The delay unit 1002 delays the signal input into the input terminal 1001, and outputs the delayed signal to the adaptive filter 1003 as a reference signal. The adaptive filter 1003 convolves the input signal (reference signal) delayed by the delay unit 1002 with a filter coefficient, so as to generate a signal having a delay of an integral multiple of a period of the howling component included in the input signal. The subtractor 1004 suppresses the howling by subtracting, from the input signal, the signal output from the adaptive filter 1003, and outputs the signal after howling suppression to the adaptive filter 1003 and the output terminal 1005. The adaptive filter 1003 sequentially updates the filter coefficient so that the mean square of the signal output from the subtractor 1004 is smallest.
As described above, the conventional howling suppression device 1000 described in Patent Literature 1 can suppress howling by extracting, using an adaptive filter, a howling component included in an input signal and subtracting the extracted howling component from the input signal.